Liquid crystals (LC) are well-known substances and their understanding is part of the general knowledge of the technical field the present invention pertains to.
A LC is composed of rod-like molecules (in some cases also having other shapes) which can be aligned so that the long directions of the rods are parallel. It is very important to be able to align the various molecules of the liquid crystal along a given axis, which can be selected according to the desired application. For example, this is essential to make modern LCD screens work.
In such a LC screen, the picture on the screen is composed of many pixels of different colors and intensities. In each pixel, the desired color is created by “mixing” blue, green and red primary colors having different intensities by means of a patterned color filter array. The intensity of each primary color is adjusted by using liquid crystals (shortly in the following “LC”) to change the light intensity transmitted from the back to the front of the display.
In the LC display, the LC is filled into a gap, a few microns wide, between two polyimide films coated onto—for example—indiumtin-oxide (ITO) electrodes which in turn are deposited onto two glass-plate cross polarizers. In order for the display to work, the LC molecules have to be anchored down nearly parallel to the surfaces of the polyimide films but on opposite sides point into the perpendicular directions of the two crossed polarizers. They thus form a twisted helix from one side to the other.
When the light from a light source in the back of the display crosses the first polarizer, it is polarized along the long axis of the LC molecules anchored to it. As the light progresses through the LC, the helical LC structure changes the polarization of the light from linear to elliptical so that part of the light is transmitted by the second, perpendicular, polarizer. Since the light transmission depends on the orientation of the LC rods, it can be changed by rotation of the rods. This is accomplished by application of a small voltage, pixel by pixel, by means of microscopic ITO electrodes independently driven by a transistor array. As the voltage is increased the LC long axis becomes increasingly parallel to the electric field direction, which is parallel to the light direction.
The light polarization becomes less affected by the LC and the light transmission is reduced because of the crossed polarizers. Thus the orientational changes in LC alignment are the heart of the LC display providing its gray scale or color contrast.
The control of the orientation of LC molecules along a given direction is important also in other fields, for example in the realization of micro-robots. A micro robot can be realized using a LC polymer structure including several parts, each part having its individual orientation within the respective volume and a characteristic dimension of the order of several microns. These integrated structures with multi-orientations can lead to functional movements, which is extremely useful for micro robotics and micro fluid systems. For instance, LC polymer micro actuator can be used for creating grippers, pumps, and switches in micro robotics or chip-on-chip systems.
Typically, in order to obtain an oriented LC structure, the latter is fabricated in a cell, often made of two glass slides positioned one opposite to the other and separated by a spacer used to define the LC film thickness. Each glass slide includes an inner surface, facing the inner surface of the opposite glass slide, and an external surface. If one or both inner surfaces of the glass slides are properly treated, so that an aligning formation is formed on or in the surface(s), after infiltrating a liquid crystal compound into the cell so that it is in contact to or in proximity of the aligning formation, the molecules forming the LC compound—at least most of them—will be spatially oriented along an aligning direction defined by the aligning formation.
Alignment “formation” is herein defined—and this definition applies to the entire text—as an element which “forces” or “induces” alignment in a portion, volume or part of the liquid crystal medium.
If the cell thickness does not exceed a certain value, for example a few hundreds of microns, the orientation present at the surfaces of the glass cell due to the aligning formation will propagate across the entire LC film thickness, in order to obtain an oriented structure as thick as the LC medium.
A method to obtain an aligning formation on the inner surface(s) of the glass slides so that such an oriented structure in the LC medium can be obtained is for example a mechanical rubbing of the surface(s) itself.
The mechanical rubbing of the surface of the glass cell is described for example in T. Ito and K. Nakanishi. “Regularity and narrowness of the intervals of the microgrooves on the rubbed polymer surfaces for LC alignment” in SID International Symposium Digest of Technical Papers, Vol XXIII, pages 393-396, Boston, Mass., USA, May 1992.
While the substrate itself (like the glass of the cell) may be processed in this way to achieve LC alignment, very often, specific alignment layers are first coated on the inner surface of the substrate (carrying already a transparent conductive layer, such as indium tin oxide or ITO), and then these specific layers are rubbed.
However, the rubbing process is not reliable and may damage the substrate or the specific layers; furthermore it may create non uniformities and dust. That is why intensive efforts are devoted to develop non-contact alignment methods of LC alignment.
Several other methods are known for aligning the molecules across the entire thickness/volume of a LC medium, such as a LC layer, besides the mechanical rubbing on the surfaces of the cell. Such methods are for example stretching of pre-prepared LC polymer films, electric or magnetic field methods where an electromagnetic field is applied to the whole LC layer to orientate the LC molecules, etc.
A photo alignment method has been described in Gibbons; Wayne M., Sun; Shao-Tang, Swetlin; Brian J. “Process of aligning and realigning liquid crystal media,” U.S. Pat. No. 4,974,941, Dec. 4, 1990; Chigrinov; Vladimir G., Kozenkov; Vladimir M., Novoseletsky; Nicolic V., Reshetnyak; Victor Y., Reznikov; Yuriy A., Schadt; Martin, Schmitt; Klaus, “Process for making photopolymers having varying molecular orientation using light to orient and polymerize”, U.S. Pat. No. 5,389,698, Feb. 14, 1995; and also a vacuum deposition method has been used as well (e.g., SiOx, as detailed in Kyung Chan Kim, Han Jin Ahn, Jong Bok Kim, Byoung Har Hwang, Hong Koo Baik, “Novel Alignment Mechanism of Liquid Crystal on a Hydrogenated Amorphous Silicon Oxide”, Langmuir 2005, 21, 11079-11084).
These methods however refer to a situation in which all molecules in the LC layer are aligned substantially along a SINGLE direction.
It is more challenging to obtain a LC alignment in which the orientation of the molecules varies from one volume to another of the same LC medium, such as a layer, in a predetermined way, i.e. a patterned orientation in multiple directions. In other words, it is technologically difficult to obtain, in a single LC sample, a non-always uniform orientation of the mesogens. However such a varying orientation, i.e. an orientation that changes depending on the position of a volume of sample taken into consideration, is extremely desirable. Further, it is preferred that these multiple orientations can be selected according to a pre-defined pattern.
Several approaches have been reported to generate micro patterns on the surface(s) of the already described cell for liquid crystals in order to obtain the desired multiple alignment. These prior art methods include for example photoalignment, ion beam irradiation, capillary force lithography, and microrubbing. All these methods are complex multistep processes.
As an example, in “Optical Patterning of multi-domain liquid-crystal displays with wide viewing angle”, written by M. Schadt et al. In Nature—Vol. 381—page 212 (16 May 1996), it is described that the successful operation of the liquid crystal displays requires control of molecular alignment, which is currently achieved by confining the liquid crystal between mechanically rubbed surfaces. But in addition to the practical difficulties associated with rubbing, the resulting displays suffer from restricted viewing angles arising from the uniaxial nature of the alignment process. This latter problem can be in principle circumvented if molecular alignment is varied, in a controlled manner, within individual pixels. Exposure to functionalized substrates to polarized light offers a mean to achieve high resolution patterns to the plane of display. But to ensure that the aligning formation, i.e. the aligning pattern imposed on the liquid crystal, is free from orientation defects, the tilt angle between the long molecular axes and the substrates must be perfectly controlled. In this article, the authors show that a linear photoalignment strategy can be extended to obtain such a control and thereby fabricate stable, multi-domain pixel displays with markedly improved fields of view.
However, these methods are extremely complex and technically challenging.